Abstract: A data transmission system having, in addition to a normal system operation mode, a test mode wherein a plurality of stations confirm each other through sending and receiving check data for security confirmation and wherein the actual line switching operation is not performed. In the test mode, when notifying test data for confirmation of another station, the call terminating station side notifies the accurate data, while the call originating station side activating the test mode notifies deliberately changed check data thereby enabling a test for confirmation of the continuance of connection with a backup line without suspending communication with the working line.